Hydrogen sulphide removal from sour gas streams is of great industrial importance, as such gases are the main known source of H2S. An important source of sour gases is synthesis gas (syngas) containing hydrogen, carbon monoxide, carbon dioxide and further components including H2S, or its subsequent product obtained by water gas shift (WGS) reaction, such as described in WO 2010/059052. The WGS reaction produces H2 and CO2 while H2S can be present in the feed stream. In Sorbent-enhanced WGS, CO2 and H2S are adsorbed onto an adsorbent such as alkali-promoted hydrotalcite and subsequently simultaneously desorbed from the adsorbent. As such, CO2 and H2S end up in the same effluent stream, restricting efficient reuse or requiring purification of such gaseous mixtures.
Known techniques for selective removal of H2S from a sour gas containing CO2 include physical, chemical and hybrid scrubbing techniques and metal oxide scavenging. Chemical scrubbing involves the use of amine-based solvents that chemically react with sour gases such as H2S and CO2. Physical solvents involve e.g. methanol or glycol, using the physical dissolution of the acid gases obeying Henry's law, and hybrid solvents combining the best of both chemical and physical solvents. Because these solvents favour H2S over CO2 only slightly, H2S enrichment yields are relatively poor, which renders this technique unsuitable for selective removal of H2S from a CO2-rich, H2S-lean stream.
EP2407227 provides a method for separating H2S from a sour syngas stream different from the aforementioned liquid absorption processes using a pressure swing adsorption system (PSA) to produce a stream enriched in CO2 and H2S, after which H2S is removed for instance by using a packed bed of ZnO that would be disposed of and replaced when saturated with H2S, or silica gels, impregnated activated carbons and/or molecular sieves. In one embodiment, steam is used to heat the bed that has been loaded with H2S to help removing said H2S. Scavengers, such as Zn-, Zn/Cu- or Fe-based scavengers, bind H2S irreversibly and thus cannot economically deal with feeds comprising relatively high amounts of H2S, such as typically 200 ppm H2S or even only 100 ppm H2S. Large scale processes or H2S levels above about 100 ppm require frequent replacement of the scavenger bed, which is usually too expensive to be economically feasible.
WO 2013/019116 discloses a process for selectively removing acidic gaseous components, in particular carbon dioxide (CO2) and hydrogen sulphide (H2S), from an adsorbent which has adsorbed these gaseous components from a feed gas. It involves a CO2 purge to replace the H2S and a subsequent H2O purge to remove the CO2. The process is well suited for a Sorption-Enhanced WGS process, which produces H2 and CO2, and wherein (small) amounts of H2S may be present. H2S and CO2 are subsequently separately separated from H2.
There remains a need for enriching a gaseous stream in H2S from a (CO2-rich, H2S-lean) feed stream that comprises intermediate amounts of H2S (e.g. 100-10,000 ppm), for which scavenger and scrubbing techniques are unsuited. Existing H2S enrichment techniques as described above can only achieve about one order of magnitude enrichment at high H2S concentrations, and two orders of magnitude increase in concentration from low H2S concentrations, for which a marked improvement is required.